Fun With Pinball

Animated Gottlieb Score Motor Pulse Masking circuits

As seen on other animated circuit pages on this site the Score Motor activates switches at specific times relative to each other during each score motor cycle, similar to the pistons in an engine each time it turns over. The basic set of switch activation times generated by the Score Motor can be combined in various ways to achieve more elaborate functions or combinations of events not generated by the Score Motor alone.

The Motor Sequence Chart

The set of switch activations generated by the Score Motor is described in a timing chart often found on the schematic diagram. Below are a typical Gottlieb Motor Sequence Chart from a 1976 Buccaneer, and for comparison, a typical Bally Sequence of Operation chart from a 1976 Aladdin's Castle.

Typical Gottlieb Motor Sequence Chart Typical Bally Sequence of Operation chart

In both charts the black boxes indicate when switches are activated as time passes and the motor rotates. Time moves from left to right across these charts so black boxes or activations on the left happen before boxes on the right. The rows indicate which switch stacks or parts of the score motor activate. On the Gottlieb chart the rows represent switch stacks mounted in various locations around the motor. Rows in the Bally chart represent the switch stacks mounted at the top of each of the motor cams (11 cams in this case).

Note that the Gottlieb chart shows just a single score motor cycle while the Bally chart shows two complete score motor cycles. Also note that sequence charts are not generic but are tailored to each game.

When a normally open switch closes it can be used to generate a brief pulse in the circuit. The black boxes in the charts above represent pulses that could be generated by normally open switches. Other types of switches however would have different effects. Normally closed switches for example would be closed at all times except during the black boxes in the charts above as we'll see below.

Circuit Description

Gottlieb often used a technique called masking to generate sets or combinations of pulses that aren't directly generated by the score motor. As an example consider a playfield target worth 30 points. The Gottlieb Score Motor has no single switch stack that could generate three pulses in each cycle while the Bally Score Motor represented by the chart above could generate three pulses using a single normally open switch on the #9 cam.

While that might seem to be a problem Gottlieb had the flexibility to generate an arbitrary number of pulses from the chart above using combinations of switches. Consider the following example circuit:

Score Motor Masking example circuit

This circuit shows a single target, the Score Motor, and a number of relay coils. When the target is hit, the normally open switch behind the target closes and starts the Score Motor which starts activating different switches. The normally open switch at the Motor 1A position generates five pulses in each Score Motor cycle which fires the "50 Points" relay coil five times during the cycle to award 50 points.

But what if the target were worth only 40 points? In that case a normally closed Motor 4C switch could be used to block the last of the five pulses from the Motor 1A switch so that only the first four pulses get through to the "40 Points" relay coil to award just 40 points.

If the target is worth only 30 points, a normally closed Motor 1B switch could be added to the circuit to block the 4th pulse, letting only the first three pulses from the Motor 1A to reach the "30 Points" relay coil which would award just 30 points.

Adding more normally closed switches could block more pulses as shown in the "20 Points" and "10 Points" circuits above.

The following animation shows how all the switches interact to fire each of the relay coils a different number of times. Note how each normally closed switch is used to block an additional pulse from the Motor 1A switch.

Score Motor masking slow

The example above uses normally closed switches to block pulses generated by the Motor 1A switch. Of course you could also use normally open switches to allow some pulses through, or to accumulate multiple pulses. That might be more appropriate if just two pulses are needed for example because it would require fewer switches to allow two pulses to pass than it would to block three pulses.

The animation below changes how the circuits for the "10 Points" and "20 Points" relay coils to demonstrate how normally open switches could be used instead to allow certain pulses to pass.

Score Motor Masking example

Overlapping Switch Activations

One thing that is important to notice in the Gottlieb Motor Sequence Chart is that the black boxes have different widths.

Gottlieb Motor Sequence Chart

There is an important reason for this. The narrow pulses from the boxes on the Motor 1A row are usually masked by the longer pulses in the other rows. For example the third Motor 1A pulse could be blocked by a normally closed switch on the Motor 4B switch stack. The Motor 4B switch however needs to open before Motor 1A switch closes, and stay open until after the Motor 1A switch reopens. Or in other words, the blocking or masking switch needs to be active longer than the switch that generates the pulse.

To understand why the masking event needs to straddle the pulse it needs to mask imagine that both switches were active for the same length of time. While it might look like that could work on the chart above, mechanical variations in the switches, the gaps between the switch contacts, the wear on the motor, or other factors could allow one to shift in time relative to the other. If the switches don't activate and deactivate at exactly the same time a small portion of the pulse might sneak through where it should have been blocked.

The chart below shows how an early or late narrow masking pulse would allow part of the pulse (in red) to slip through while a wider pulse at the bottom would be more tolerant of variations or changes in timing.

Masking pulse comparisons

Animated Schematic Diagrams

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